1. Field of the Invention
The present invention relates to a method for testing respective characteristics of a plurality of switching elements arranged on an OLED (Organic Light Emitting Diode) substrate and an OLED display equipped with means for correcting non-uniform characteristics of each switching element.
2. Description of Related Art
An OLED display has an OLED substrate having elements and wiring arranged on an insulating substrate. More particularly, in an OLED display, a plurality of switching elements and OLED elements are arranged on an insulating substrate in the form of matrix. Each switching element has a selection terminal and a data terminal. The selection terminals of the switching elements arranged in rows are connected to one selection signal line. The data terminals of the switching elements arranged in columns are connected to one data signal line. There are a plurality of selection signal lines and data signal lines, which cross each other. An OLED display emits OLED elements selected by each switching element to show the OLED elements. Thin film transistors (TFT) are used as switching elements.
Amorphous silicone and polysilicon are used as semiconductor materials for these thin film transistors. Thin film transistors made from amorphous silicon do not have so remarkable nonuniformity in characteristics, so that unevenness of display is dispersed to the whole screen. On the contrary, polysilicon made by local heating with laser irradiation tends to have non-uniform characteristics per unit of pixel.
These switching elements cause non-uniform luminance in OLED elements due to non-uniform characteristics. As a result, display quality is uneven. It follows that stabilizing characteristics of switching elements is important.
However, the number of switching elements is dramatically increased when the display size gets bigger. Consequently, it is difficult to improve characteristics of all switching elements.
Patent document 1 discloses a display for correcting luminance by measuring a load current of OLED elements in a current measurement circuit and correcting displayed data using its measurements.
Further, patent document 2 discloses a display for improving display quality by obtaining uniform quantity of emitted light per pixel and OLED substrate.
Patent document 1: Japanese Patent Publication No. 2002-341825
Patent document 2: Japanese Patent Publication No. 10-333641
Displays targeted for displaying moving picture become extremely deteriorated in picture quality when nonuniformity of emitted light luminance is locally concentrated on the screen. Thin film transistors using amorphous silicon which tend to disperse nonuniformity in characteristics onto the whole screen are employed as switching elements. Switching rate and threshold voltage are important out of the characteristics of switching elements because luminance nonuniformity occurs when the threshold voltage is dispersed.
The display disclosed in patent document 1 detects a current passing through switching elements, that is, OLED elements to control the signal level provided to data signal lines based on this detected current.
An additional circuit is, however, needed between an OLED substrate and a driver integrated circuit (IC) for providing data signal lines with data. An additional circuit comprises a current measuring part, a memory for storing measured current values, an operation part for operating the quantity to be corrected, a D/A converter for converting digital data outputted from the operation part to analog data. An additional circuit is required for each data signal line. This raised a problem of high costs for additional circuits.
The display disclosed in patent document 2 also needs a picture signal memory for storing a picture signal, an arithmetic circuit for processing a signal from the memory, a plurality of sub-frame memories for storing a plurality of sub-frame data outputted from the arithmetic circuit, and a read out circuit for reading particular frame data from the sub-frame memories or the like. Accordingly, there was a problem that the circuit of the display became complicated.
Moreover, the display needs a long test time for testing all pixels and a large-capacity memory to store data.